“Guys think about sex every five seconds,” says Justin Burkhardt, 21, of Allentown, who's single. “Guys definitely think about sex more than girls.”

Nothing unites men and women more than sex. Yet nothing divides them more either. It all starts with a single chromosome: the male-making Y, a puny thread bearing a paltry 25 genes, compared with the lavish female X, studded with 1,000 to 1,500 genes.

But the Y guy, containing a gene dubbed Sry, circulates masculinity via a fabled hormone called testosterone. And that hormone not only masculinizes the body, but affects the developing brain and wiring of nerve cells.

“It's a reasonably safe assumption that young males are driven by sex,” says Gregg Amore, a psychologist and director of counseling at DeSales University. “Some men's brains are below their belt.”

Males and females differ in mating psychology because our minds are shaped by our reproductive mandates. And, in men, this mandate means a more casual attitude toward sex.

“Women want to know sex means something; men don't necessarily need that,” says Rod Broker, 28, of Allentown, who's single. “Women always see it as an attachment, but it doesn't always need to be. It's not that guys are insensitive. To them, it's just a physical act. To women, it's an emotional act.”

Various studies back up the all-sex-all-the-time phenomenon in men. Traditionally for them, love is one thing, and sex is... well, sex.

“The 17-year-old is the most testosterone-driven beast on Earth-completely motivated by sex,” says Abraham Nemitz, 28, of Lower Saucon Township, who's married. “It's completely ironic. Society would never allow it to happen, but a 40-year-old female and a 17-year-old male would be a great sexual match.”

With neither matrimony nor monogamy getting in the way, our lucky cave-dwelling ancestors were free to engage in as much no-hang-up sex as they wanted.

“Nothing is ever true 100 percent, but for the majority of men, it's the thrill of the hunt,” says Matt Sommerfield, 28, of Allentown, who's married. “It's an innate drive.”

Some experts believe our anthropological legacy may shed light on exasperating male behavior ranging from commitment-phobia to pornography to cheating. And yet, both experts and regular guys admit an enlightened society must transcend animal nature.

“The biological explanation definitely makes my students more comfortable. They think, 'Oh, it's just my hormones,'” says Dr. Jeremy Teissere, assistant professor of biology and director of the neuroscience program at Muhlenberg College. “But a lot of biological and social forces get in the way of understanding the male sex drive in isolation. Sex drive is absolutely biological, but it's so complicated, so messy.”

Take three typically male sports: wandering eyes, locker-room-talk and cheating. All have upset women for ages. Yet men say cutting them slack on at least two of the three can go a long way in alleviating misunderstandings.

Exhibit A: Wandering eyes.

“It's natural to do that. I find myself doing it and say, 'What am I doing?'” says Michael Drabenstott, 37, of Allentown, who's married with children. “It's part of being a guy. Women shouldn't feel threatened by it. I mean, if a guy is salivating and panting, that's one thing, but as long as it isn't harmful, women should be understanding.”

All the men interviewed for this story agree:

“For a guy, it's as natural as breathing or blinking,” says Sommerfield. “But it's on the surface, meaningless, guys certainly mean no harm by it, they don't do it intentionally.”

“Women think guys are dogs,” says Burkhardt. “But it's just a pretty girl walking by. Women misunderstand. It does depend on the guy, though. Some are subtle. Others holler and yell and can be disrespectful.”

“It's silly that either gender can't appreciate beauty,” says Amore.

Exhibit B: Locker-room-talk.

“It's the way guys talk to each other. It's just the way men relate,” says Broker. “I don't know any guy who says, 'Please don't say that. I'm offended.'”

Exhibit C: Cheating.

While men admit cheating can possibly be explained by biology — mixing with as many mates as possible to ensure gene survival — they all agree it's unacceptable.

“Cheating in any form is wrong. It's dishonest, particularly in matrimony,” says Amore. “It's not justified even if men have a higher level of testosterone.”

Twenty-five percent of wives and 44 percent of husbands cheat, according to Psychology Today, but in an intriguing dwindling of the double standard, the gap is closing.

“I think cheating is related to sex drive but it isn't an excuse at all,” says Burkhardt. “You can put those urges aside. It shows a weak will. They can easily be curbed.”

“Cheating is partially biologically-driven but it's socially unacceptable. You have to make an intelligent decision even though your body is telling you something else,” says Drabenstott. “Just because it's biology doesn't mean you have to go through with it. If you can't quell it, that's a sign of weakness.”

“It's wrong to use biology as some kind of excuse,” says Teissere. “We think it absolves us when, in reality, it's a social phenonemon.”

“The view that all men are driven by sex is a disservice to men. There are guys like me who want to get married, who don't have commitment issues,” says Nemitz. “There's a small percentage of unabashed skirt-chasers who make us all look bad. They damage every woman out there for the rest of us!”

Men who are older and/or married generally believe sex is just one of many drives motivating men.

“When you're a teenager, it drives everything, but it wanes as you get older,” says Jerrell Sharp, 56, of Quakertown, who's engaged after two previous marriages. “But it's a good thing that you become more relaxed about your sexuality. You're so preoccupied with it when you're younger that it can get you into relationships that aren't a good match.”

Drabenstott agrees. “After you settle down, other priorities become more important, as opposed to looking for action all the time. If sex is a nine or ten when you're in your 20s, and everything else is a four or five, sex is what you're going to focus on. But if sex is still a nine or ten when you're in your 30s, and your kids, wife and job are a nine and ten too, you're going to focus on it a little less. But it will always be an important part of men's lives, because it starts out at such a high level, even if it wanes over time.”

Teissere says the sexual behaviors of chimpanzees in the wild tend to muddy the whole sex drive debate anyway.

“Both genders have high sex drives and it's hard for both of them to be monogamous,” he says of the primates.

Teissere believes, rather, that societal expectations drive the sexual bus. “There's nothing about aggression or sex that's particularly masculine. I think women definitely feel uncomfortable with a hyped-up sex drive. Females can't have a strong sex drive without some social stigma. Women who are open and honest about sex are seen as whorish.”

And even though men may be from Mars and women may be from Venus, every man interviewed for this story thinks it isn't hormones that cause differences, but the lack of communication about them.

“Boys are taught to suck it up — be a man,” says Amore. “It's whining from a male perspective. They're conditioned not to emote.”

But, girlfriends, don't despair. Men do have feelings!

“Women should know that when men don't communicate, it doesn't mean they have nothing to communicate,” says Drabenstott. “They're afraid or they don't know how to articulate it. Men have things to say, but they don't know how to say them. Women need to help men open up and men need to accept help from women to open up.”

And, yes, men do fall deeply in love.

“It's not that men don't want to be with one person,” says Broker. “It just has to be the right person.”

Burkhardt agrees. “If it's the right girl, guys love to be in a relationship.”

Ultimately, what's at stake in the battle of the sexes is more important than mere romance.

“There's no question society today offers rewards for feminine-type behaviors. Women are perceived as more nurturing, empathetic, understanding,” says Amore. “Society has evolved as women entered the workforce. The military model — nose to the grindstone — doesn't really work anymore. Work today is about teamwork, fun, a softer approach. There's a much higher level of consciousness in how we treat people.”

Some call this brave new world the “feminization” of society, and it's a place where good old boys need not apply.

“You can use raw force on the playground, but it's not the way to resolve disputes in the corporate world,” says Nemitz. “It's not as advantageous to men — when the man's role and woman's role used to be more clear — but is that unfair to men? I don't think so. It's a loss for men, but not necessarily a loss for society.”

Sharp thinks traditionally male instincts should just be channeled better.

“Aggression and dominance shouldn't be used in an antisocial way. But they can be used in a positive way, a protective way. That same drive leads me to protect my family,” he says.

And men are often undervalued, he says, for the little and big things they do every day.

“When it's bad weather, who traditionally drives? The man. When there's some nasty, ugly thing to do...who does it? The man. When there's danger to be faced...who does it? The man.”

Stamping out male aggression has consequences, Sharp says.

“You can't have protection and breed passivity at the same time, but by the same token, you can't let it go wild. Maybe it shouldn't be tamed, but trained.”

Men are definitely confused about their place in this new world.

“There are mixed messages out there,” says Amore. “On the one hand, there are athletes who are paid to be strong and aggressive and, to some degree, violent. And then we say to boys, 'Okay, now let's sit down and talk about your feelings.' There's a lot of confusion in the male population.”

Could men one day become unnecessary, as a new book claims?

“All things mutate-biological systems, government systems, financial systems,” says Sharp. “Maybe men's traditional role in society is superfluous. Certainly, women today can have a life without men.”

So, in a world where women don't need men for financial security, raising children or even having sex, where does that leave them?

“Men can offer women love and companionship, even though those aren't traditional roles for men,” says Sharp. “Emotionally fulfilling relationships is something men have to work on. I don't think being emotionally available is a female trait. I think women just got there before men.”